BUILD: makefile: don't disable -Wstringop-overflow anymore

This basically reverts commit c4e6460f6 ("MINOR: build: Disable
-Wstringop-overflow.") which is no more needed after previous one.
diff --git a/Makefile b/Makefile
index 3bfc84b..d4c96a2 100644
--- a/Makefile
+++ b/Makefile
@@ -185,7 +185,6 @@
 SPEC_CFLAGS += $(call cc-nowarn,unused-parameter)
 SPEC_CFLAGS += $(call cc-nowarn,clobbered)
 SPEC_CFLAGS += $(call cc-nowarn,missing-field-initializers)
-SPEC_CFLAGS += $(call cc-nowarn,stringop-overflow)
 SPEC_CFLAGS += $(call cc-nowarn,cast-function-type)
 SPEC_CFLAGS += $(call cc-nowarn,string-plus-int)
 SPEC_CFLAGS += $(call cc-opt,-Wtype-limits)